The present invention relates to a diffusing film, which can be attached on z screen side in a liquid crystal display. In detail, the invention relates to a diffusing film and a polarizing plate that improve quality of an image given by a liquid crystal display.
A liquid crystal display generally comprises a polarizing plate and a liquid crystal cell. For ensuring the quality of displayed images, the viewing angle must be enlarged and the display surface (screen) must be prevented from reflecting light coming from outside.
The most widely used display is a TFT liquid crystal display of TN mode, in which an optical compensatory sheet is provided between the polarizing plate and the liquid crystal cell. As described in Japanese Patent Provisional Publication Nos. 7(1995)-191217, 8(1996)-50206 and European Patent No. 0911656A2, the TFT liquid crystal display of TN mode gives an image of high quality within a wide viewing angle range. However, gradation of an image displayed by the TFT liquid crystal display-is often inverted when the screen is seen downward.
To solve the problem, various proposals have been given. For example, it has been proposed that a diffusing means (Japanese Patent No. 2,822,983), an optical axis exchanger (Japanese Patent Publication No. 2001-33783) or an optical means for diffusing emitted light (Japanese Patent Publication No. 2001-56461) be provided on the observer side surface to improve the image quality remarkably. However, in these methods, it is necessary to use a diffusing means having highly controlled lens or diffraction grating structure, which costs a lot and is very difficult to mass-produce.
Japanese Patent Provisional Publication Nos. 6(1994)-18706 and 10(1998)-20103 disclose a diffusing film in which a transparent substrate is coated with a resin containing filler (e.g., silicone dioxide (silica)). That diffusing film can be mass-produced at low cost.
Japanese Patent Provisional Publication Nos. 11(1999)-160505, 11(1999)-305010, 11(1999)-326608, 2000-121809, 2000-180611 and 2000-338310 disclose other diffusing films, but they cannot fully improve the image quality.
Recently, liquid crystal displays have been often used as monitors having enough fine pixels to display minute images. However, the monitors equipped with the diffusing films often display blurred images.
For preventing the display surface from reflecting light coming from outside, an anti-reflection film is generally provided. The anti-reflection film is placed on the top surface of the display, and causes optical interference to reduce reflectance so that the displaying screen may be prevented from reflecting surrounding scenes and thereby so that the contrast of displayed image may not impaired. However, the anti-reflection film by no means solves the problem of viewing angle (inversion of gradation), and therefore it has been wanted to provide an anti-reflection film which prevents well the display surface from reflecting light coming from outside and at the same time which solves the problem of viewing angle.
It is an object of the present invention to provide a diffusing film and an anti-reflection film for enlarging the viewing angle of liquid crystal display, particularly for preventing the display surface from reflecting surrounding scenes and for preventing inversion of gradation when the display is seen downward. The aimed films enable a liquid crystal display to give a clear image even if the display is used as the minutely displaying monitor.
It is another object of the invention to provide a polarizing plate that enlarges the viewing angle (particularly, viewing angle when the display is seen downward) without thickening the liquid crystal display. The aimed polarizing plate has improved durability, and prevents the image quality from impairments (such as deterioration of contrast, inversion of gradation or inversion of black to white or white to black, and unfavorable fluctuation of hue), which are often observed according to the viewing angle. The invention also aims to provide a liquid crystal display equipped with that polarizing plate.
The objects of the invention are achieved by the diffusing films (1) to (12), the processes for preparation of a diffusing film (13) to (15), the polarizing plates (16) to (18) and the liquid crystal displays (19) and (20), described below.
(1) A diffusing film comprising a transparent resin in which scatterers are dispersed, wherein the difference between the refractive index of the transparent resin and that of the scatterers is in the range of 0.04 to 1.5, and wherein scatterers are flat particles having particle sizes in the range of 0.1 to 50 xcexcm.
(2) The diffusing film of (1), wherein the scatterers are tabular particles.
(3) The diffusing film of (2), wherein the ratio of the average size of scatterers in a section parallel to the film plane to that in a section perpendicular to the film plane is 2 or more.
(4) The diffusing film of (1), wherein the scatterers are columnar or rod-like particles.
(5) The diffusing film of (4), wherein the ratio of the average size of scatterers in the section parallel to the film plane to that in the section perpendicular to the film plane is 0.5 or less.
(6) The diffusing film of (1), wherein at least one surface of the diffusing film has a surface roughness of 0.05 to 0.18 xcexcm.
(7) The diffusing film of (1), wherein the transparent resin further contains a matting agent in the form of fine particles, and the difference between the refractive index of the transparent resin and that of the matting agent is less than 0.04.
(8) The diffusing film of (1), wherein the transparent resin furthermore contains superfine particles whose particle sizes are less than 0.1 xcexcm.
(9) The diffusing film of (1), wherein a scattering layer comprising the transparent resin and the scatterers dispersed therein is provided on a transparent substrate.
(10) The diffusing film of (9), wherein the scattering layer has a thickness of 2.0 to 6.0 xcexcm.
(11) The diffusing film of (9), wherein a low-refractive index layer is further provided on the scattering layer.
(12) The diffusing film of (11), wherein an incident ray coming into the low-refractive index layer at the incident angle of 5xc2x0 is reflected in an average mirror reflectance of 2.5% or less in the wavelength region of 450 to 650 nm.
(13) A process for preparation of a diffusing film comprising the steps of: dissolving or dispersing a transparent resin and scatterers whose refractive index differs from that of the transparent resin by 0.04 to 1.5, in a mixture of a solvent having a function of dissolving a transparent substrate and another solvent having a function of not dissolving the substrate, to prepare a coating solution; applying the solution onto the transparent substrate; and drying the solution to form a scattering layer containing flat scatters which have particle size of 0.1 to 50 xcexcm.
(14) The process of (13), wherein the scattering layer has a thickness of 2.0 to 6.0 xcexcm, and the transparent resin soaks into a depth of 0.01 to 1.0 xcexcm in the substrate.
(15) The process of (13), wherein the substrate is made of cellulose acetate, and the solvent dissolving the substrate contains a ketone.
(16) A polarizing plate comprising a pair of protective films and a polarizing membrane provided between them, wherein one of the protective films is a diffusing film comprising a transparent resin in which scatterers are dispersed, wherein the difference between the refractive index of the transparent resin and that of the scatterers is in the range of 0.04 to 1.5, and wherein scatterers are flat particles having particle sizes in the range of 0.1 to 50 xcexcm.
(17) The polarizing plate of (16), wherein an optically anisotrbpic layer comprising a liquid crystal compound is provided on the other protective film.
(18) The polarizing plate of (17), wherein the liquid crystal compound is a discotic compound.
(19) A liquid crystal display of transmission type comprising a pair of polarizing plates and a liquid crystal cell provided between them, wherein the polarizing plate comprises a pair of protective films and a polarizing membrane provided between them, and the protective film on the screen side is a diffusing film comprising a transparent resin in which scatterers are dispersed, wherein the difference between the refractive index of the transparent resin and that of the scatterers is in the range of 0.04 to 1.5, and wherein scatterers are flat particles having particle sizes in the range of 0.1 to 50 xcexcm.
(20) A liquid crystal display of reflection type comprising a reflection board, a liquid crystal cell and a polarizing plate in this order, wherein the polarizing plate comprises a pair of protective films and a polarizing membrane provided between them, and the protective film on the screen side is a diffusing film comprising a transparent resin in which scatterers are dispersed, wherein the difference between the refractive index of the transparent resin and that of the scatterers is in the range of 0.04 to 1.5, and wherein scatterers are flat particles having particle sizes in the range of 0.1 to 50 xcexcm.
The applicant has found that the scattering profile (intensity according to viewing angle) of light having passed through the scattering layer within a particular angle to the normal and the haze value thereof (particularly, the internal scattering haze value thereof, which greatly contributes to diffusion of the transmitted light) relate to improvement of the viewing angle character and to blur of the displayed image. In detail, according to the applicant""s study, the intensity of light scattered within the angle of 40xc2x0 to the normal enlarges the viewing angle, and that at an angle of 50xc2x0 or more makes the blur worse.
The applicant has further studied, and finally succeeded in controlling the surface roughness by properly selecting shapes (sectional sizes seen at the observing angle) of the scatterers. The surface roughness is controlled so that the desired scattering profile can be obtained. Further, a low-refractive index layer is formed on a surface having the properly controlled roughness to prepare an anti-reflection film, which can prevent the displaying screen from reflecting surrounding scenes and also which can improve the viewing angle character without blurring the displayed image (consequently, the transmitted image can be kept very clear).
Light emitted from the backlight is scattered by the diffusing film provided on the polarizing plate of the observer side. The more the light is scattered within the angle of 40xc2x0 to the normal, the more the viewing angle character is improved. However, if too much amount of light is scattered at the angle of 50xc2x0 or more to the normal, the blur becomes worse. Further, if the light scattered at the angle of 90xc2x0 or more to the normal (light scattered backward) increases, the brightness of image seen frontally is lowered or the clearness of image is impaired.
Although conventional spherical scatterers can improve the viewing angle character, they at the same time make the blur worse or impair the brightness of image seen frontally. In contrast, according to the invention, the viewing angle character can be improved without impairing the quality of displayed image (without lowering the brightness and without causing the blur).